Reticulated and controlled porosity battery structures

ABSTRACT

The effective ionic conductivity in a composite structure is believed to decrease rapidly with volume fraction. A system, such as a bipolar device or energy storage device, has structures or components in which the diffusion length or path that electrodes or ions must traverse is minimized and the interfacial area exposed to the ions or electrons is maximized. The device includes components that can be reticulated or has a reticulated interface so that an interface area can be increased. The increased interfacial perimeter increases the available sites for reaction of ionic species. Many different reticulation patterns can be used. The aspect ratio of the reticulated features can be varied. Such bipolar devices can be fabricated by a variety of methods or procedures. A bipolar device having structures of reticulated interface can be tailored for the purposes of controlling and optimizing charge and discharge kinetics. A bipolar device having graded porosity structures can have improved transport properties because the diffusion controlling reaction kinetics can be modified. Graded porosity electrodes can be linearly or nonlinearly graded. A bipolar device having perforated structures also provides improved transport properties by removing tortuosity and reducing diffusion distance.

This application is a continuation of U.S. patent application Ser. No.12/041,619, filed Mar. 3, 2008 now U.S. Pat. No. 7,781,098, which is adivisional of U.S. patent application Ser. No. 10/021,740, filed Oct.22, 2001 now U.S. Pat. No. 7,553,584, which claims priority to U.S.Provisional Patent Application Ser. No. 60/242,124, filed Oct. 20, 2000,the contents of which applications are incorporated herein by referencein their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to bipolar devices, and moreparticularly, to batteries having electrodes that are reticulated, orinterdigitated, a controlled porosity, and to those that are perforated.

2. Description of the Related Art

Solid state energy devices, such as but not limited to lithium batteriesor fuel cells, typically require high energy density as well as highpower density. Power density can be related to the discharge rate, whichcan be a function of ion and electron transport rates. For example, anelectrode in a lithium battery that is too thick can limit dischargerate because ion/electrode transport from the electrode to the interfacewith a separator, such as the electrolyte, can be rate limiting. On theother hand, if the electrode layers are very thin, then energy densitysuffers because the electrolyte, separator, and current collectorsoccupy a higher volume and contribute to a greater mass relative to theactive material of the electrodes. In addition, the discharge rate canbe limited by interface resistance allowing only a certain current rateper unit area of interface.

The lithium-ion and lithium-polymer rechargeable battery can be anattractive technology for rechargeable battery applications due to itshigh energy density, freedom in battery configuration, low potential forenvironmental and safety hazard, and low associated materials andprocessing costs.

Improvements in lithium rechargeable battery technology have occurreddue to improvements in the storage materials used as the cathodes oranodes, or in the liquid or polymer electrolytes used with suchbatteries. Currently known cathode compounds such as LiCoO₂ and LiMn₂O₄when used with currently known anodes such as lithium metal or carbonhave working voltages between about three and four eV. For manyapplications a high voltage and low weight are desirable for the cathodeas this leads to high specific energy. For example, for electricalvehicle applications the energy-to-weight ratio of the batterydetermines the ultimate driving distance between recharging.

Research into lithium intercalation compounds that has been conductedthus far has focused primarily on the synthesis and subsequent testingof various oxide compounds. These efforts have led to the development ofa variety of compounds, including Li_(x)CoO₂, Li_(x)NiO₂, Li_(x)Mn₂O₄,and Li_(x)V₃O₁₃. In addition, Li_(x)TiS₂ and other disulfides have beeninvestigated for use in lithium intercalation.

Systems with multiple metals have been described in several patents andpublications. Ohzuku, et al., “Synthesis and Characterization ofLiAl_(1/4)Ni_(3/4)O₂ for Lithium-Ion (Schuttle Cock) Batteries,” J.Electrochem. Soc., vol. 142, p. 4033 (1995), and Chiang et al., “HighCapacity, Temperature-Stable Lithium Aluminum Manganese Oxide Cathodesfor Rechargeable Batteries,” Electrochem. Sol. St. Lett., 2(3) pp.107-110 (1999) describe the mixed-metal composition of the title andreport electrochemical properties thereof. Cathodes in some rechargeablelithium batteries contain lithium ion host materials, electronicallyconductive particles to electronically connect the lithium ion hosts toa current collector (i.e., a battery terminal), a binder, and alithium-conducting liquid electrolyte. The lithium ion host particlestypically are particles of lithium intercalation compounds, and theelectronically conductive particles are typically made of a substancesuch as carbon black or graphite. The resulting cathode includes amixture of particles of average size typically on the order of no morethan about 100 microns.

Anodes for rechargeable lithium-ion batteries typically contain alithium ion host material such as graphite, a binder, and a lithiumconducting liquid electrolyte. Alternatives to graphite or other carbonsas the lithium ion host have been described by Idota et al., in Science1997, 276, 1395, and by Limthongkul et al., in “Nanocomposite Li-IonBattery Anodes Produced by the Partial Reduction of Mixed Oxides,” Chem.Mat. 2001.

In such cathodes or anodes, for reliable operation, good contact betweenparticles should be maintained to ensure an electronically-conductivepathway between lithium host particles and the external circuit, and alithium-ion-conductive pathway between lithium host particles and theelectrolyte. To that, flooded electrolyte batteries have been used.Flooded electrolyte batteries are generally those wherein the electrodesare immersed in an electrolyte solution or matrix. This should improveperformance by providing additional reaction sites.

Energy density can be intrinsically determined by the storage materials;the cell voltage can be determined by the difference in lithium chemicalpotential between cathode and anode, while the charge capacity candepend on the lithium concentration that can be reversibly intercalatedby the cathode and anode. Power density, on the other hand, can be atransport-limited quantity, determined by the rate at which ions orelectrons can be inserted into or removed from the electrodes.

Solid polymer electrolytes have been described. For example, Nagaoka, etal., in “A High Ionic Conductivity in Poly(dimethyl siloxane-co-ethyleneoxide) Dissolving Lithium Perchlorate,” Journal of Polymer Science:Polymer Letters Edition, Vol. 22, 659-663 (1984), describe ionicconductivity in poly(dimethyl siloxane-co-ethylene oxide) doped withLiClO₄. Bouridah, et al., in an article entitled, “aPoly(dimethylsiloxane)-Poly(ethylene-oxide) Based Polyurethane NetworksUsed as Electrolytes in Lithium Electrochemical Solid State Batteries,”Solid State Ionics, 15, 233-240 (1985) describe crosslinkedpolyether-grafted PDMS filled with 10 wt % LiClO₄, and its ionicconductivity and thermal stability. Matsumoto, et al., in an articletitled, “Ionic Conductivity of Dual-Phase Polymer Electrolytes Comprisedof NBR-SBR Latex Films Swollen with Lithium Salt Solutions,” J.Electrochem. Soc., 141, 8 (August, 1994) describe a technique involvingswelling poly(acrylonitrile-co-butadiene) rubber andpoly(styrene-co-butadiene) rubber mixed latex films with lithium saltsolutions resulting in dual-phase polymer electrolytes.

Electrodes for polymer batteries have also been described. For example,Minett, et al. in “polymeric insertion electrodes, Solid State Ionics,28-30, 1192-1196 (1988)” describe a mixed ionic/electronic conductingpolymer matrix formed by exposing a film of polyethylene oxide soaked inpyrrole to aqueous FeCl₃ solution or by exposing a film ofFeCl₃-impregnated polyethylene oxide to pyrrole vapor. Films wereassembled into all-solid-state electrochemical cells using lithium asthe anode and PEO₈LiClO₄ as electrolyte. U.S. Pat. No. 4,758,483(Armand) teaches of a solid polymeric electrolyte that can be used in acomposite electrode. The electrolyte can include an ionic compound insolution in a copolymer of ethylene oxide and a second unit that can bean ethylene polyoxide structure including side-group radicals thatintroduce structural irregularities into the system reducing oreliminating crystallinity. A lithium salt, such as lithium perchlorate,can be dissolved in the polymer system.

While significant advances in battery formulations have been made, thereis much room for improvement in increased power density and energydensity in these types of devices.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention provides an energy storagedevice comprising at least one reticulated electrode in ionic contactwith an electrolyte.

In another embodiment, the present invention provides an energy devicecomprising a first electrode having features defining a plurality ofextensions into an electrolyte matrix.

In another embodiment, the present invention provides a bipolar device.The bipolar device comprises a first electrode having a first set ofproturberances extending into an electrolyte and a second electrodehaving a second set of protuberances constructed and arranged to becomplementary to the first set of protuberances.

In another embodiment, the present invention provides an electrodecomprising a framework having a porous network defined therein.

In another embodiment, the present invention provides a battery. Thebattery comprises a first electrode, a second electrode, a first currentcollector in electronic communication with the first electrode and asecond current collector in electronic communication with the secondelectrode. The first electrode includes a portion, positioned betweenthe first current collector, and a second electrode, having a porositythat increases in a direction from the first current collector towardthe second electrode.

In another embodiment, the present invention provides an apparatuscomprising a first electrode having a first mating surface forpositioning proximate an opposing electrode, the mating surfacereticulated so as to define a plurality of protrusions and interveningindentations providing a surface area at least 1.5 times the theoreticalsurface area of the first mating surface in a smooth, non-reticulatedconfiguration.

In another embodiment, the present invention provides an electrodecomprising a plurality of channels defined therein and constructed andarranged to allow diffusion of an ionic species from an electrolyte to asurface thereof.

In another embodiment, the present invention provides a batterycomprising an electrode in contact with an electrolyte and having aplurality of channels defined therein, the channels constructed andarranged to allow diffusion of an ionic species from the electrolyte toa surface thereof.

In another embodiment, the present invention provides a batterycomprising at least one perforated electrode in ionic communication withan electrolyte.

In another embodiment, the present invention provides a bipolar devicecomprising a porous electrode that is free of polymer binder.

In another embodiment, the present invention provides a bipolar devicecomprising a porous electrode that is free of carbon additive.

In another embodiment, the present invention provides a method forfacilitating providing energy. The method comprises the step ofproviding a battery comprising a first electrode, a second electrode, afirst current collector in electronic communication with the firstelectrode and a second current collector in electronic communicationwith the second electrode, wherein the first electrode includes aportion, positioned between the first current collector and the secondelectrode, having a porosity that increases in a direction from thefirst current collector toward the second electrode.

Other advantages, novel features, and objects of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings, which areschematic and which are not intended to be drawn to scale. In thefigures, each identical, or substantially similar component that isillustrated in various figures is represented by a single numeral ornotation. For purposes of clarity, not every component is labeled inevery figure. Nor is every component of each embodiment of the inventionshown where illustration is not necessary to allow those of ordinaryskill in the art to understand the invention.

BRIEF DESCRIPTION OF DRAWINGS

Preferred, non-limiting embodiments of the present invention will bedescribed by way examples with reference to the accompanying figures, inwhich:

FIG. 1 is a schematic illustration showing an anode/cathode system thatcan be used in accordance with the present invention;

FIG. 2 is another schematic diagram illustrating another embodiment ofthe present invention illustrating simulated cells;

FIGS. 3A-3D are schematic (cross-section) illustrations showing bipolardevices with various reticulated electrodes according to anotherembodiment of the present invention;

FIG. 4 is a schematic illustration showing a bipolar device having aperforated structure according to another embodiment of the presentinvention;

FIG. 5 is a graph showing electrolyte volume fraction as a function ofdistance in an electrode according to one embodiment of the presentinvention;

FIG. 6 is a graph predicting a normalized cumulative ionic resistance ina greater porosity structure in a bipolar device according to oneembodiment of the present invention;

FIG. 7 is a graph showing a normalized cumulative potential drop in agreater porosity structure in a bipolar device according to oneembodiment of the present invention;

FIG. 8 is a graph showing the specific energy of a greater porositystructure as a function of current density in a bipolar device accordingto one embodiment of the present invention;

FIG. 9 is a graph showing the specific energy as a function of specificpower in a bipolar device according to one embodiment of the presentinvention;

FIG. 10 is a graph showing the specific energy as a function ofelectrolyte fraction at the surface of a graded porosity structure in abipolar device according to one embodiment of the present invention; and

FIG. 11 is a graph showing the specific energy as a function ofdischarge current density in a bipolar device having a graded porositystructure according to one embodiment of the present invention.

DETAILED DESCRIPTION

To improve the intrinsic transport properties of electrochemicallyactive oxides, a three-phase porous electrode can be used to improve therate-limitation. A carbonaceous conducting additive and an electrolytematerial can be added to the storage material, lithium, cobalt oxide,for example, to improve the electronic and ionic conductivity.

Typically, microstructural features control the critical properties ofsuch materials. Accordingly, the microstructure of components in suchsystems is tailored to optimize desirable properties and minimize theundesirable ones.

A bipolar device according to one embodiment of the present invention isschematically depicted in FIG. 1. The bipolar device 10, which can be anenergy storage system, can use, in one embodiment, a LiCoO₂/carboncombination. In some cases, a solid polymer energy storage system, suchas a battery, can be provided and comprises an electrolyte 16, lithiummetal anodes 12, and cathodes 14. Energy storage devices according tothe present invention, such as but not limited to lithium ion batteries,can be based on liquid electrolytes. For example, the typical lithiumbattery has a lithium foil or a composite carbon anode, a liquidelectrolyte with a lithium salt and a composite cathode. Duringdischarge, lithium ions move through the electrolyte from the anode tothe cathode, and then intercalate into the oxide storage material. Topreserve charge neutrality, electrons are driven through the externalcircuit 18 to complete the electrochemical reaction. Preferably, theelectrode should provide fast transport for both electrons and lithiumions.

Realizable energy and power density is typically influenced by systemdesign, including, for example, component arrangement and selection.Typical high-performance rechargeable energy storage systems are oflaminate construction, and can use composite electrodes that aretypically a mixture of active material, binder, and conductive additive.The system can be flooded with organic liquid electrolyte. The thicknessof the cathode in a lithium-ion battery is typically less than 200 μm,and for high power batteries, less than 100 μm. To maximize the packingdensity of storage material, for high energy density, the pore channelscan be made to be tortuous and limited in cross-sectional area. It isbelieved that the rate-limiting transport step is, in most instances Li⁺ion diffusion, through the liquid-filled pore channels of the compositeelectrode. Currently the “cell stack” can be two metal foil currentcollectors, anode, separator, and cathode, that is about 250 μm thick.

A lithium ion battery will be used to generally describe the variousaspects of the present invention. The description of such a lithium ionbipolar device is meant to be exemplary and the use of the variousfeatures and aspects of the present invention to other systems isconsidered to be within the scope of the present invention. For example,the below described reticulated, perforated or controlled porositystructures can be used for energy storage or energy conversion systemsincluding but not limited to primary (disposable) and secondary(rechargeable) batteries.

The lithium battery can be charged by applying a voltage between theelectrodes 12 and 14, which causes lithium ions and electrons to bewithdrawn from lithium hosts at the battery's cathode. Lithium ions flowfrom cathode 14 to anode 12 through electrolyte 16 to be reduced at theanode. During discharge, with reference to FIG. 1, the reverse occurs;lithium ions and electrons enter lithium hosts 20 at cathode 14 whilelithium can be oxidized to lithium ions at anode 12, which is typicallyan energetically favorable process that drives electrons through anexternal circuit 18, thereby supplying electrical power to a device towhich the battery is connected. Thus, during battery operation, forexample, lithium ions pass through several steps to complete theelectrochemical reaction. Typically, the steps include, dissolution oflithium at the anode surface, which typically releases an electron tothe external circuit; transport of the lithium ions through theelectrolyte (which can reside in pores of a separator and, with porouselectrodes, in the electrodes' pores) separator, for example, theelectrolyte; transport of the lithium ions through the electrolyte phasein a composite cathode; intercalation into the active cathode material,which typically receives electrons from the external circuit; anddiffusion of lithium ions into the active material along with electrontransport from a current collector to the intercalation sites.

The lithium dissolution at the anode and the intercalation reaction atthe cathode-electrolyte interface can be thermally activated and can begenerally characterized by reaction kinetics. The charge transferreactions, typically at the electrodes, are believed to be relativelyfast at room temperature and, thus, not necessarily rate-limiting.Nevertheless, such reactions can be accelerated by increasing thesurface area of the reaction. Reducing the particle size of theintercalation material can increase the rate of reaction. Ionintercalation into an electrode can be characterized by diffusion. Fortypical intercalation oxides at room temperature, the diffusion time, ina typical energy storage device, across a typical distance of about onegm can be about ten seconds. Notably, diffusion limitations can bereduced by reducing the oxide particle size but can be addressed byaltering other diffusion parameters.

Ion transport across the separator 16 typically occurs in two regions,the separator region 22 and the electrode region 24. In the former,generally, no electrochemical reactions occur and transport phenomenacan be governed by the separator physical properties. The rateassociated with this phenomenon can be reduced by designing oroptimizing separator physical properties or by minimizing the transportdistance across the separator. In the latter, ion transport can occurthrough the electrolyte-filled pore channels or network structures ofthe electrode. The ion transport can be affected by, for example, thetortuosity of the average ion transport path. In some systems, the ioniccurrent changes with electrode depth because of the electrochemicalreaction.

The effective ionic conductivity in a composite structure 12 or 14 isbelieved to decrease rapidly with decreasing pore volume fraction, saidpores being filled with electrolyte. Accordingly, in one embodiment, thepresent invention provides an electrode structure 12 or 14 that favorsor promotes ion transport. For example, according to one embodiment, thepresent invention provides a system comprising lamellar particlesarranged to be substantially parallel to the direction of current flow.With such a lamellar microstructure, the volume fraction of activematerial can be increased without reducing ionic conductivity.

According to another embodiment, the present invention provides abipolar device 10 having a design in which the current collector andelectrolyte mass is minimized while the anode and cathode structuresmass are maximized In one embodiment, the diffusion length, d, or paththat electrodes or ions must traverse is minimized and the interfacialarea exposed to the ions or electrons is maximized.

That is, in one embodiment, the system can include components orstructures that can be reticulated or has a reticulated interface sothat an interface area can be increased. In this way, the increasedinterfacial perimeter increases the available sites for reaction of, forexample, ionic species. Many different reticulation patterns can be usedaccording to the present invention including the reticulated structuresshown schematically in FIGS. 3A-3D. In one embodiment, the aspect ratiol/a of this feature can be varied where l is the length of a protrusion(or indentation), described below, and a is its width or thickness. Sucha bipolar device can be fabricated by a variety of methods orprocedures, as described below. FIGS. 3A-3D show systems having avariety of reticulated structures. In FIG. 3A, system 10 has areticulated anode 12 having a plurality of extensions 28 extending intoand in ionic communication with electrolyte matrix 16. In thisembodiment, cathode 14 is shown as non-reticulated. Similarly, accordingto another embodiment, FIG. 3B shows system 10 having a reticulatedanode 12 and a reticulated cathode 14, each having protrusions 28 andcomplementary indentations 26 that are separated from each other at auniform distance. Anode 12 and cathode 14 can be in ionic and/orelectronic communication with electrolyte 16. In FIG. 3 c, system 10 hascomplementary reticulated structures 12 and 14, each beinginterdigitated, the reticulations having a length, l, and a width orthickness, a. In FIG. 3C, system 10 has reticulated structures 12 and14, each in electronic communication with a current collector 30. Thereticulations form convexities 28 that are at a separation distance, d,from correspondingly-shaped concavities 26.

In addition to producing a single layer cell, or a stack, a multilayercell with a higher energy density and power density can be achieved withthe same materials in a planar interface design. The present inventionprovides systems or cells with a wide range of properties, for example,discharge rates, power densities, that can be made of the same set ofmaterials. This provides flexibility and can lead to a more efficientdesign, prototyping and manufacturing sequence, as well as providing atailorable or customizable bipolar device. A bipolar device havingstructures of reticulated interface can be tailored for the purposes ofcontrolling and optimizing charge and discharge kinetics.

In the present invention, “reticulated interface” or “interdigitatedelectrode” refers to a battery 10 that has a structure, such as apositive and/or a negative electrode 12 and 14 each of which can beconnectable to a current collector 30 everywhere, including cases wherethe positive and negative electrodes serve as their own currentcollector and having a morphology such that the surface exposed isreticulated, having convexities 26 or protrusions 28 and,correspondingly, concavities or indentations, sufficient to producefeatures with a thickness or width that is less than the maximumthickness or width of each electrode. Such features may be periodic andregularly spaced or aperiodic or random. The morphology of thestructures exhibit shape complementarity towards one another such thatwhere one electrode has a protrusion, the other tends to have aindentation of similar shape and dimension. The positive and negativeelectrode can be separated everywhere along their “mating” interface bya layer or region of electrolyte 16. In some embodiments, especiallywith respect to systems with shape complementary structures, the layerof electrolyte 16 can be thin and can have a relatively uniformthickness.

It is preferred that the spatially-averaged thickness of the layer ofelectrolyte or separator between positive and negative electrodes beless than about 100 microns, preferably less than about 50 microns,still preferably less than about 25 microns, and still preferably lessthan about 10 microns. It is also preferred that the reticulatedfeatures of the positive and negative electrode have a thickness, whenaveraged along the length of the protrusion or indentations, that isless than about 100 microns, preferably less than about 50 microns,still preferably less than about 25 microns, and still preferably lessthan about 10 microns. Such designs can decrease the volume of thesystems that would normally be consumed by the separator, electrolyte,binder, conductive additive, and other inert components that, in someembodiments, do not store lithium, and thereby increases the energydensity of the battery on a volume or weight basis.

Having the above stated dimensions, this design also has improved poweron a volume or weight basis compared to batteries of conventionaldesign, because the ion diffusion distance can be decreased. In aconventional laminated battery design in which the thickness of thepositive and negative electrodes are approximately uniform, duringcharging or discharging the ions must diffuse across the thickness ofthe electrodes. In a conventional lithium ion device, the electrodethickness is typically about 100 to about 200 micrometers. In most suchsystems the rate of transport of lithium ions across the electrodethickness limits the power. The transport rate of electrons is believedto be much higher and is not necessarily rate-limiting. In the presentinvention, when applied to a lithium ion battery, the lithium iondiffusion distance can be decreased, from a value equal to the electrodethickness to a value equal to the lateral dimensions of the reticulatedor interdigitated features.

In another embodiment, the present invention provides increasing theinterfacial area between an electrode of a bipolar device and aseparator or electrolyte to reduce the diffusion distance or to minimizethe length of diffusion paths. In some cases, as shown in schematicallyin FIG. 4, the present invention provides a system 10 having aperforated structure, such as an electrode 12 and 14, that has aplurality of channels 32 defined therein. In one embodiment, theplurality of channels can be filled with electrolyte material. Such astructure can improve ionic diffusion by minimizing diffusiontortuosity. Thus, the effective diffusion length can be decreased. Insome cases, perforated preferred electrodes can be used as a compositecathode in lithium ion batteries. In another embodiment, the presentinvention provides a thin film battery wherein the electrode can be adense single phase material that has a plurality of channels filled withsolid electrolyte 16. The right side of FIG. 4 shows a cross-sectionalong a-a of electrode 14. The cross-section shows electrolyte 16 in thechannels 32 of electrode 14. The channels can extend through and acrossthe electrode, from the front at interface 34 with separator 16 to theback near current collector 30. Channels 32 provide ionic communicationbetween the back of the electrolyte and the region near the back of anelectrode. This alternate transport path should reduce the transportdistance by removing tortuosity that an ionic species may travel.Channels 32 can have a variety of cross-sectional shapes such as, butnot limited to circular, as shown in FIG. 4, rectangular or polygonal.

The present design can also provide a system wherein the charge ordischarge characteristics can be selectively tuned by altering thedimensions of the reticulated or interpenetrating features.Microfabrication approaches such as those described below allow thesefeature shapes and dimensions to be readily varied thus providingimproved control over system characteristics without relying on the typeof material. This improves design, prototyping, and manufacturing,compared to conventional energy storage systems where materialsformulations are typically empirically adjusted to achieve desiredproperties. In another embodiment, the present invention providesimproved ion transport in a composite structure, such as an electrode,by adjusting the ionic conductivity relative to the current distributionin the structure. When a charge transfer current in the electrodeparticles is rate-limiting, the current carried by the electrolyte phasein the electrode can decrease with depth. Such a phenomenon typicallyindicates that the ionic conductivity of the electrolyte phase near theregion away from the electrolyte separator may not be critical while ahigh ionic conductivity near the electrode surface requires rapid iontransport towards the bulk of the electrode structure. Accordingly, inone embodiment, the present invention provides improved transport ratesby grading the porosity, or porosity density, of the electrodestructure. A high volume fraction of electrolyte near the interface,with the bulk electrolyte, can improve ionic conductivity in the regionwhere ion current can be high, to improve rate capability, while ahigher fraction of the active material in the depth of the electrodeallows retaining a high energy density.

The present invention provides a variety of graded porosity arrangementsincluding, but not limited to, linear, concave up and concave downporosity gradients. An electrode, for example, with a linear porositygradient typically has a continuously, or at least a non-discretely,varying porosity from one region to another region. For example, anelectrode can have a linearly varying porosity, filled with electrolyte,in one embodiment, so that a porosity of 0.4 can be at the front 36 ofthe electrode, near the electrolyte, and a porosity of 0.2 can be at theback 38 of the electrode, near the current collector. The back refers tothe region of an electrode that is in electronic communication with acurrent collector and the front refers to the region of an electrodethat is positioned adjacent the separator electrolyte. In otherembodiments, the electrode has a porosity that can have concave up orconcave down profile.

The porosity can average from about 10% to about 70%. It is believedthat if the porosity is too high, above about 80%, then the frameworkmay be structurally unstable; if the porosity is too low, below about10%, then there is only an incremental increase in power or energydensity. Accordingly, the average porosity is, preferably from about 20%to about 50%. In another embodiment, the average porosity is from about30% to about 45%. In some embodiments, the porosity gradient in anelectrode, from the current collector toward the electrolyte or theother electrode, varies by at least about 10% from the average porosity,preferably, at least about 20%, more preferably, at least about 30%. Inother embodiments, at any cross-section of an electrode perpendicular toa line connecting the center of mass of the current collector and thecenter of mass of the other electrode, the porosity variation is uniformto about +/−10%, preferably about +/−5%, more preferably, about +/−3%,and even more preferably about +/−1%.

Thus, the system can have structures that have a porous network in aframework. The porous network can be ionically interconnected so thations can diffuse to the cavities defining the porosity at any locationwithin the porous structure. For example, a lithium ion can diffuse fromthe bulk electrolyte to any ionically interconnected location in aporous electrode.

These graded porosity gradients are graphically illustrated in FIG. 5.In FIG. 5, the average porosity is about 0.3 and each of the gradedporosity electrodes has a porosity of about 0.4 at the front of theelectrode, which corresponded to an electrolyte fraction of 0.4.

The performance of the bipolar system shown in the figures relates to atypical LiMn₂O₄ spinel cathode with a EC/DEC/LiPF₆ electrolyte andeither a MCMB carbon or lithium anode schematically illustrated in FIG.2. The mesoporous carbon microbeads (MCMB) carbon anode was used forevaluations of graded porosity electrodes. For discharges, a spinelcathode was assumed with an initial lithium content of Li_(0.1705)Mn₂O₄.The systems were simulated to be discharged to a cutoff of about 3.5 V.The cathode thickness was assumed to be about 200 μm; the electrolytethickness was assumed to be about 52 μm and the anode thickness wasassumed to be about 100 μm. In the figures, various gradients are shownfor an average porosity of 0.3.

FIG. 6 is a graphical illustration of the normalized cumulative ionicresistance as a function of electrode depth for each of the gradedporosity electrodes shown in FIG. 5. Each of the graded porosityelectrodes had a predicted lower cumulative ionic resistance than aconventional electrode near the surface and throughout the electrode.FIG. 7 is a graphical illustration of the normalized cumulativepotential drop as a function of electrode depth for each of the gradedporosity electrodes shown in FIG. 5. Each of the graded porosityelectrodes has a lower potential drop than a conventional electrode nearthe surface as well as throughout the electrode. FIGS. 6 and 7 show thatthe graded porosity electrode has better ionic transport and potentialproperties that should translate to higher power and energy densities.Such performance can be graphically illustrated in FIGS. 8 and 9, whichshow, respectively, the specific energy relative to the current densityand specific power, for a variety of graded porosity electrodes. FIG. 9shows that the systems with graded porosity electrodes would supply moreenergy at a given power than a conventional electrode system. Moreover,FIG. 10, which is a graphical illustration of the specific energy as afunction of porosity (electrolyte fraction at the electrode surface),shows that as the discharge current increases, the optimum electrodegrading shifts from a slight porosity to more severe gradients at highcurrent densities. It is believed that the shift follows from decreasingelectrode utilization with increasing current where lower ion transportproperties at the back of the electrode, especially for highly gradedelectrodes, inhibits utilization at low and moderate discharge rates.FIG. 11, which is a graphical illustration of specific energy as afunction of discharge current density for systems with concave up,concave down and linearly gradient porosity electrodes, shows that thegraded porosity systems have higher specific energy compared to aconventional, homogeneous electrode system, especially at theintermediate discharge rate regime.

In accordance with another embodiment, the electrode has a porositygradient, from the current collector to the other electrode or theelectrolyte, that has a slope that varies by less than or no more than5% at any location along the electrode, preferably, by less than or nomore than 10%, more preferably, by less than or no more than 15%. Thechange in slope can be stepwise or smooth.

In another embodiment, the structures have a mating surface that isreticulated with a surface area that is at least 1.5 times thetheoretical surface area of a smooth, non-reticulated structure,preferably, the reticulated surface area is at least about 2.5 times,more preferably, at least about 3 times, even more preferably, at least4 times, and most preferably, at least about 5 times.

In another embodiment, the reticulations have an aspect ratio that is atleast about 2, preferably at least about 2.5, more preferably at leastabout 3.0, more preferably at least 3.0, more preferably at least about4.0, and most preferably, at least about 5.0.

In another embodiment, the protrusions and indentations are separated byan average distance of less than about 100 microns. Preferably, theseparation distance is less than about 50 microns, more preferably, lessthan 25 microns, most preferably, less than about 10 microns.

The function and advantage of these and other embodiments of the presentinvention will be more fully understood from the examples below. Thefollowing examples are intended to illustrate the benefits of thepresent invention, but do not exemplify the full scope of the invention.

EXAMPLES Prophetic Example 1 Lithium Battery Prepared by SequentialDeposition

A suspension can be prepared of a fine powder lithium storage cathodesuch as LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, LiFePO₄, V₂O₅, Li₃Bi, Li₃Sb, orother such cathodes well-known to those skilled in the art, in a solventwith a binder, optionally a conductive additive such as carbon, andother additives well-known to impart characteristics to the suspensionallowing it to be deposited in thin layers using stenciling, screenprinting, inkjet printing, or lithographic methods selected to allow alateral resolution to the printed layer that is within the desireddimensional ranges. A separate like suspension can be prepared of a finepowder lithium storage anode such as carbon, Sn, Sb, Al, Zn, Ag, LiAl orother anode materials known to those skilled in the art. The cathodesuspension and anode suspension are deposited layer by layer, providinga periodic or aperiodic reticulated or interdigitated structure asdescribed above and as shown in FIG. 3. Electronic contact, shorting,between the cathode and the anode is avoided by selecting the solventand binder system such that a continuous (wetting) surface layer of thebinder forms upon drying, and/or by depositing the layers such that,within the same layer, cathode patterns and anode patterns areadequately separated. Optionally, a third suspension containing binderand no cathode or anode or conductive additive can be deposited in apattern at the interface of the cathode and anode patterns to ensureelectronic isolation of the two.

A metal foil or fine mesh current collector made of, for example,aluminum or copper, can be used as the substrate upon which layers aredeposited. Aluminum is preferred when the cathode compound forms a firstcontinuous layer and copper is preferred when the anode forms a firstcontinuous layer. After sequential deposition is complete, and theassembly is dried and, optionally, heated for consolidation, a secondcurrent collector can be applied to the surface of the layered battery.Optionally, the top current collector is formed by printing as aconductive ink using techniques such as those used for forming patternedinterconnects as those used by those skilled in the art of electronicdevice fabrication. Optionally, the battery is deposited on aninsulating film such as, but not limited to, polyethylene or polyestersuch as MYLAR® film, available from the E.I. du Pont de Nemours andCompany (Wilmington, Del.), from which the battery can be subsequentlyremoved and current collectors can be applied to form contacts with theanode and cathode.

The binder is, for example, a solid polymer electrolyte. This shouldobviate the need for liquid electrolyte in the battery, and, in someinstance, serves to bind the particles securely together in theassembled device while allowing liquid electrolyte to be infused(flooded) throughout the battery. An example of suitable solid polymerelectrolyte includes, is not limited to, (poly)ethylene oxide in which alithium salt such as lithium perchlorate or lithium triflate has beenadded. An example of a binder and liquid electrolyte that remainsdimensionally stable, i.e., the electrolyte does not dissolve thebinder, is (poly)ethylene difluoride (PVdF) and ethylenecarbonate-dimethyl carbonate (EC:DMC) in a 1:1 molar ratio to which alithium salt has been added.

Prophetic Example 2 Battery Produced by Printing and Coating

A first electrode with a reticulated or interdigitated structure, eithercathode or anode, is prepared using the materials and methods ofExample 1. At the free surface of the printed structure, a continuousfilm of a binder or polymer electrolyte can be formed. The film can forma physical separator between anode and cathode. The film can be formedby self-segregation (wetting) of the binder solution to the free surfaceof the printed electrode. Optionally, the surface film can be formed bycoating with a liquid binder or electrolyte solution followed by drying,or by vapor deposition techniques known to those skilled in the art ofthin film materials preparation.

A conformal coating of a liquid suspension can be applied to the formedstructure to create the counter electrode. The indentations of thelatter fill in complementary fashion to the structure of the firstelectrode, leaving a smooth and flat outer surface to which a currentcollector is subsequently applied. Multiple coatings may be used toachieve conformal filling. The system can then be dried and optionallyheated for consolidation. A current collector can be applied to one orboth surfaces to complete the system.

Prophetic Example 3 Battery Produced by Embossing and Coating

A layer of a first electrode, either cathode or anode, formulated of thematerials and by the methods of Example 1, is cast or coated in a layerupon a metal foil current collector or an insulating film. This layer isformulated by methods known to those skilled in the art to haverheological characteristics appropriate for thick film processing, forexample, by screen printing, tape casting, web coating, and similarprocesses. The surface of the first layer is then embossed with a die toleave a reticulated surface with dimensions as desired. To this shapedsurface is applied a counterelectrode by the conformal coating materialand process described in Example 2. The assembly is dried and optionallyheated for consolidation and a current collector is applied.

A film of binder or electrolyte is applied before or after the embossingstep, and before coating with the counterelectrode formulation.

Prophetic Example No. 4 Subtractive Patterning Followed by Filling

A layer of a first electrode, either cathode or anode, formulated of thematerials and by the methods of Example 1, is cast or coated in a layerupon a metal foil current collector or an insulating film. Optionallythe electrode is cast or coated as a suspension upon a metal foilcurrent collector and fired to obtain a continuous solid film of thestorage material, or deposited as a solid film by a vapor depositionprocess known to those skilled in the art, such as sputtering,evaporation, chemical vapor deposition. The layer of first electrode issubtractively patterned, that is, material is removed, to form thereticulated or interdigitated electrode topology of the invention, bylithographic masking followed by chemical or reactive-ion etching, laserremoval, or other such methods known in thick and thin film materialsprocessing. Upon the patterned first electrode is optionally deposited afilm of binder or electrolyte, followed by coating with thecounterelectrode so as to conformally fill the pattern in the firstelectrode, by the method of Example 3.

Prophetic Example 5 Graded Porosity Electrode Produced by DifferentialSedimentation

It is well-known to those skilled in the art of powder processing thatthe Stokes' settling rate of particles in a fluid is a function of thesize and shape of the particles, the difference in density between theparticle and the fluid within which it is settling, and the fluidviscosity. For the same particle material, smaller particles tend tosettle slower than larger particles, and anisometric particles such asrods of large length to diameter ratio, or plates of large width tothickness ratio, settle at a slower average rate than spheres orequiaxed particles of identical volume. It is furthermore known thathighly aspected particles tend to settle to a lower packing density thanequiaxed particles of the same material. Therefore a method forintroducing a porosity gradient into a layer of storage electrodefabricated from a powder mixture or suspension is use a mixture ofparticle sizes and shapes.

A suspension is made of a cathode oxide powder in which the powdercontains a distribution of particle sizes and shapes. Equiaxed particlesare mixed with platelet-shaped particles, with the particles sizesselected such that the equiaxed particles have a higher Stokes' settlingvelocity. The powder is formulated with a binder (such as PVdF), a fineconductive additive (such as high surface area carbon) and a solvent toproduce a castable suspension. The suspension is formulated to allowdifferential sedimentation of the cathode oxide particles within a fewminutes to a few hours after casting a film from the suspension. Thefilm is cast, printed, or coated on a metal foil current collector or aninsulating film, whereupon differential sedimentation occurs under theforce of gravity resulting in a higher packing density of equiaxedparticles in the portion of the electrode adjacent to the metal currentcollector, and a lower packing density of anisometric particles awayfrom the metal current collector. This introduces a desired porositygradient in the electrode. After drying, the electrode is laminated witha separator and a counterelectrode and infused with organic liquidelectrolyte to produce a battery cell. Optionally, a cathode oxide withhigh electronic conductivity, such as LiMg_(0.05)Co_(0.95)O₂, is usedand no carbon additive is used.

A graded porosity carbon anode is produced in like manner, using carbonpowder selected to have a mixture of equiaxed particle shapes andanisometric particles shapes, as well as differences in density thatallow the Stokes' settling rates to be adjusted. In one instance MCMBare used as the equiaxed carbon particle which settles more rapidly andforms a more densely packed region adjacent to the current collector,and flake graphite with platelet particle shape is used as theanisometric carbon particle which settles more slowly and forms thelower packing density region adjacent to the separator. The porositygradient is adjusted by selecting the relative amounts of the particleforms and the size of the MCMB and flake graphite particles.

Prophetic Example 6 Graded Porosity Electrode Produced by DifferentialSedimentation of a Fugitive Filler

In this example, a suspension is used to form a cast, printed, or coatedlayer of electrode as in Example 5. However, the electrode storagematerial is mixed in the suspension with one or more additional solidmaterials which upon heating are removed, thereby leaving behindporosity. Therefore the solid material that is removed is a “fugitive”pore former. The density, particle size and size distribution, andparticle shape of the electrode storage material and the fugitive poreformer are selected to provide a differential Stokes' settling rategiving in the final product a more densely packed storage materialadjacent to the current collector, and less densely packed storagematerial adjacent to the separator.

In one instance the storage material is an oxide cathode such as LiCoO2,LiMg_(0.05)Co_(0.95)O₂, LiMnO₂, or LiFePO₄. The fugitive pore former isMCMB, selected to have a particle size giving a slower Stokes' settlingrate than the cathode oxide. A suspension is prepared containing thesetwo solids as well as a solvent and optionally a binder, the specificformulation being selected to allow differential sedimentation of thecathode oxide and MCMB. The suspension is cast, printed, or coated on ametal current collector and fired in an oxidizing ambient that pyrolysesthe MCMB and sinters the cathode oxide to form a connected layer. Thesintered porous cathode layer contains a desired porosity gradient oncethe MCMB has been removed.

In another instance, the fugitive pore former consists of particles ofan organic or inorganic compound with a melting point between about 0°C. and 800° C. The preparation of the suspension and the casting processare carried out below the melting point of the compound. Subsequently,the cast, printed, or coated film is heated above the melting point ofthe organic compound allowing it to be drained or evaporated from theporous electrode, leaving a desired porosity gradient.

In still another embodiment, the fugitive pore former is a solid with ahigh vapor pressure, such as napthalene, and which is removed bysublimation rather than melting, leaving a desired porosity gradient.

Those skilled in the art should appreciate that all parameters andconfigurations described herein are meant to be exemplary and thatactual parameters and configurations will depend upon the specificapplication in which the systems and methods of the present inventionare used. Those skilled in the art should recognize, or be able toascertain, using no more than routine experimentation, many equivalentsto the specific embodiments of the invention described herein. Forexample, the selection and sizing of the channels in perforatedelectrodes is considered to require no more than routineexperimentation. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described. The presentinvention is directed to each feature, system, or method describedherein. In addition, any combination of two or more features, systems ormethods, if such features, systems or methods are not mutuallyinconsistent, is considered to be within the scope of the presentinvention. For example, the use of channels in reticulated electrodes orthe incorporation of a porosity gradient with perforated or reticulatedelectrode is considered to be within the scope of the present invention.

1. An electrode comprising a framework having an ionicallyinterconnected porous network, wherein the porous network has a porositythat continuously varies from a first end to a second end, defining aporosity gradient, such that the porosity gradient exhibits a linear, aconcave up, or a concave down profile from the first end to the secondend.
 2. The electrode of claim 1, wherein the porosity at the first endis at least 10% less than an average porosity of the electrode.
 3. Theelectrode of claim 2, wherein the porosity at the second end is at least10% greater than the average porosity of the electrode.
 4. The electrodeof claim 1, wherein the porosity varies from the first end to the secondend by more than about 5%.
 5. The electrode of claim 1, wherein theporosity gradient exhibits a linear profile.
 6. The electrode of claim1, wherein the porosity gradient exhibits a concave up profile.
 7. Theelectrode of claim 1, wherein the porosity gradient exhibits a concavedown profile.
 8. An energy storage device, comprising: a firstelectrode; a second electrode; a first current collector in electroniccommunication with the first electrode; a second current collector inelectronic communication with the second electrode; and an electrolytein ionic communication with the first and second electrodes, wherein atleast one of the first and second electrodes includes a framework havingan ionically interconnected porous network, wherein the porous networkhas a porosity that continuously varies from a first end to a secondend, defining a porosity gradient, such that the porosity gradientexhibits a linear, a concave up, or a concave down profile from thefirst end to the second end.
 9. The energy storage device of claim 8,wherein the porosity gradient exhibits a linear profile.
 10. The energystorage device of claim 8, wherein the porosity gradient exhibits aconcave up profile.
 11. The energy storage device of claim 8, whereinthe porosity gradient exhibits a concave down profile.
 12. A batterycomprising: a first electrode; a second electrode; a first currentcollector in electronic communication with the first electrode; and asecond current collector in electronic communication with the secondelectrode; wherein the first electrode includes a framework having anionically interconnected porous network, wherein the porous network hasa porosity that continuously varies from a first end to a second end,defining a porosity gradient, such that the porosity gradient exhibits alinear, a concave up, or a concave down profile from the first end tothe second end.
 13. A battery as in claim 12, further comprising aporous separator separating the first electrode and second electrode,the battery constructed and arranged to receive a liquid electrolytepermeating the separator and at least a portion of the porous portion ofthe first electrode.
 14. A battery as in claim 12, wherein the first andsecond electrodes each include a porous portion adapted to receive aliquid electrolyte, each of the first and second electrodes having aporosity that increases in a direction toward the other electrode.
 15. Abattery as in claim 12, wherein the first electrode has a porous portionwith an average porosity of from about 10 to about 70%.
 16. A battery asin claim 12, wherein the first electrode has a porous portion with anaverage porosity of from about 20 to 50%.
 17. A battery as in claim 12,wherein the first electrode has a porous portion with an averageporosity of from about 30 to 45%.
 18. A battery as in claim 12, thefirst electrode having a porous portion with an average porosity and aporosity gradient in a direction from the first current collector towardthe second electrode, wherein the porosity at each extreme of thegradient is at least 10% different from the average porosity.
 19. Abattery as in claim 12, the first electrode having a porous portion withan average porosity and a porosity gradient in a direction from thefirst current collector toward the second electrode, wherein theporosity at each extreme of the gradient is at least 20% different fromthe average porosity.
 20. A battery as in claim 12, the first electrodehaving a porous portion with an average porosity and a porosity gradientin a direction from the first current collector toward the secondelectrode, wherein the porosity at each extreme of the gradient is atleast 30% different from the average porosity.
 21. A battery as in claim12, wherein the porosity of any cross section of the first electrodeperpendicular to a line connecting the center of mass of the currentcollector and the center of mass of the second electrode is uniform to+/−10%.
 22. A battery as in claim 12, wherein the porosity of any crosssection of the first electrode perpendicular to a line connecting thecenter of mass of the current collector and the center of mass of thesecond electrode is uniform to +/−5%.
 23. A battery as in claim 12,wherein the porosity of any cross section of the first electrodeperpendicular to a line connecting the center of mass of the currentcollector and the center of mass of the second electrode is uniform to+/−3%.
 24. A battery as in claim 12, wherein the porosity of any crosssection of the first electrode perpendicular to a line connecting thecenter of mass of the current collector and the center of mass of thesecond electrode is uniform to +/−1%.
 25. A battery as in claim 12,wherein the first electrode has a porosity gradient in a direction fromthe first current collector toward the second electrode having a slopethat varies by no more than 5% at any location within the firstelectrode.
 26. A battery as in claim 12, wherein the first electrode hasa porosity gradient in a direction from the first current collectortoward the second electrode having a slope that varies by no more than10% at any location within the first electrode.
 27. A battery as inclaim 12, wherein the first electrode has a porosity gradient in adirection from the first current collector toward the second electrodehaving a slope that varies by no more than 15% at any location withinthe first electrode.
 28. A battery as in claim 12, wherein the porositygradient exhibits a linear profile.
 29. A battery as in claim 12,wherein the porosity gradient exhibits a concave up profile.
 30. Abattery as in claim 12, wherein the porosity gradient exhibits a concavedown profile.